AROS Root Disease

Description

Silvicultural treatments for the control of Armillaria root-disease in Oregon

Introduction: The purpose of this project was to determine if silvicultural treatments on sites severely infected with Armillaria root disease (caused by Armillaria ostoyae) were effective for reducing growth losses and mortality in susceptible host species and promoting the regeneration of more Armillaria-tolerant tree species in mixed-conifer forests in eastern Oregon.

In 1990, plots were established on a 400-acre, uneven-aged mixed-conifer forest on the Fremont-Winema National Forest (Klamath Ranger District).  The vegetation of the study site consisted of scattered 200-800 year old Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa), an overstory dominated by 50-150-year-old white fir (Abies concolor), and a sparse understory of mostly white fir seedlings and saplings.  The entire site was heavily infected with Armillaria ostoya. Plots were established at the frequency of 1 plot per acre, on a systematic grid.  Each plot consisted of a variable-radius plot (basal area factor 28, 34, or 40) and a ?xed-radius circular plot (0.01 acre) centered at the same point. Pre-treatment data collected in 1990 included stand elevation, aspect, slope, inoculum index, root-disease-severity rating, and density index.  Data collected for living trees within plots included: species, diameter at breast height (dbh; to the nearest 0.1 in.), number and size of all stem wounds >0.1 ft² in area that penetrated the inner bark, and condition (healthy-appearing or live-infected with Armillaria, Heterobasidion, or Echinodontium tinctorium). Seedlings were tallied by species and condition.  Only trees >5.0 inches dbh were tallied in the variable-radius plots, while only trees <5.0 inches dbh and >6 inches tall were tallied in the ?xed-radius plots.  Data recorded for all dead trees >0.1 inches dbh within plots included the species, dbh, years dead, and cause of death.

In1995, two silvicultural treatments (shelter-wood and group-selection harvesting) were applied, with the method of harvesting designated based on stand conditions.  The intent of the treatments was to harvest as many of the dead and dying trees as possible, leaving Armillaria-tolerant species (Douglas-?r, pine, Incense cedar [Calocedrus decurrens], and Western larch [Larix occidentalis and create sufficient openings for successful natural and artificial  regeneration  of  seedlings of these more tolerant species. In 1996, ponderosa pine, sugar pine (P. lambertiana) resistant to blister rust (Cronartium ribicola), incense-cedar, and Douglas-?r were planted in areas opened by both harvesting treatments.

Plots were surveyed immediately after treatments (1995) and 10 years later in 2005.  In the post-treatment surveys, data were collected as in the pre-treatment survey.  In 2005, additional dbh-growth data were also collected.  Due to the sparse distribution of seedlings and saplings other than white fir, small trees (2 ft height but <5.0 in. dbh) were sampled in 2007 in 0.5–1.5-ac openings made by harvesting, with one to three 0.25-ac circular fixed-area plots per opening (with  the same centers as the original plots).

Results: Ten years post-treatments, Armillaria-caused mortality was greatest for Shasta red ?r (A. magnifica var. shastensis) and white ?r (38% and 31% of trees per acre, respectively) and much less in Douglas-?r and ponderosa pine (3% and 0% of trees per acre, respectively).

No significant difference was found for either leave-tree mortality or the diameter-growth response between treated and untreated plots. It is possible that the benefits gained by thinning treatments were reduced due to factors such as compacted or displaced soils caused by thinning operations, and/or 10 years may be too short a period to detect significant differences in growth and mortality between treatments.

Over the period of this study, the primary benefits observed for reducing the impacts of Armillaria root disease were the shift in the species composition from infection-prone true ?r species to more Armillaria-tolerant Douglas-?r and pine, and the increased frequency of more Armillaria-resistant seedlings and saplings (including ponderosa pine, lodgepole pine (P. contorta), sugar pine, Douglas-?r, Western larch, and incense-cedar) in treated stands.

More detailed descriptions and results from this project are available in (Filip et al. 2010)

Literature cited:

Filip, G. M.; H. M. Maffei; K. L. Chadwick; T. A. Max, 2010: Armillaria Root Disease-Caused Tree Mortality following Silvicultural Treatments (Shelterwood or Group Selection) in an Oregon Mixed-Conifer Forest: Insights from a 10-Year Case Study. Western Journal of Applied Forestry, 25, 136-143.